This invention relates to recovering oil from a subterranean reservoir by displacing oil into a production well by injecting foam forming components through an injection well. More particularly, the invention relates to improving the efficiency with which an oil displacing foam is formed throughout most, if not all, of the reservoir interval between injection and production wells.
Numerous processes for recovering oil by injecting foam-forming components into oil-containing subterranean reservoirs have been described in patents such as the following: U.S. Pat. No. 3,269,460 describes injecting liquid containing dissolved gas which bubbles when the pressure is reduced as the liquid moves away from the injection well or encounters a zone of high permeability. U.S. Pat. No. 3,318,379 describes injecting a surfactant solution, then surfactant-free liquid, then gas, so that foam formation occurs relatively far from the injection well. U.S. Pat. No. 3,342,256 describes injecting surfactant solution not later than injecting CO.sub.2, then injecting an aqueous liquid, so that thief zones within the reservoir become plugged by foam. U.S. Pat. No. 3,412,793 describes injecting steam and surfactant to form temporarily stable steam foam plugs within thief zones. U.S. Pat. No. 3,464,491 describes injecting foaming agent and gas to form foam plugs in thief zones to improve an underground combustion drive by preventing bypassing flows of air through the thief zones. U.S. Pat. No. 3,491,832 describes injecting alternating slugs of surfactant and gas and using surfactant-free liquid slugs between them to increase the distance of penetration of the foam. U.S. Pat. No. 3,529,668 describes injecting alternating liquid and gas slugs of a specified size behind an aqueous surfactant solution. U.S. Pat. No. 3,893,511 describes recovering oil from reservoirs having interconnected very high and very low permeabilities by injecting surfactant and oil-soluble gas to foam in the permeable zones and divert gas into the oil so that oil is displaced into the permeable zones, breaks the foam in those zones, and flows into producing locations when the pressure in the producing locations is reduced to significantly less than injection pressure. U.S. Pat. No. 4,086,964 describes a steam drive process, for recovering oil from reservoirs susceptible to steam channel formation, by circulating through a steam channel a mixture of steam and foam forming surfactant arranged to increase the pressure gradient within the channel without plugging the channel. U.S. Pat. No. 4,113,011 describes using a specified organic sulfate surfactant at a pressure greater than 1500 psi in an oil recovery process like that of U.S. Pat. No. 3,342,256.
Thus, it appears that the prior art teaches that foams are capable of displacing oil, are capable of plugging permeable zones--and how it may be difficult to cause a foam having such capabilities to be (a) formed within a subterranean reservoir at a significant distance away from an injection well or (b) formed around the injection well and then transmitted through the reservoir.
However, as far as applicants are aware, the prior art suggests nothing regarding the possibility of solving such a foam distribution problem by cyclically lowering the production well pressure while continuing to inject the foaming components. When a mixture of surfactant and gas is injected into a reservoir and is being displaced through the pores of the reservoir, it is known that a forming or strengthening of foam may occur when the mixture encounters a zone of reduced pressure, such as a fracture or highly permeable streak. Such a foam formation or strengthening is said to occur in cyclic stimulation or soak-type oil production operations in which foam components are injected and fluid is produced from a single well or in pressure cycling processes such as those of U.S. Pat. No. 3,893,511, which use an oil-soluble gas to recover oil from "dead-end" pores of a dual permeability reservoir.